The present invention relates to a laser pulse amplifier device being adapted for providing high average power laser pulses. Furthermore, the invention relates to a method of amplifying laser pulses. Applications of the invention are present in the fields of laser techniques, e.g. for measuring purposes or for creating radiation pulses by high harmonic generation (HHG).
Conventionally, the standard technique for providing high average power laser pulses having a duration in the ps- or sub-ps-range at repetition rates in the kHz regime is based on a combination of a pulse stretcher, a pulse amplifier and a pulse compressor (so-called chirped pulse amplification, CPA). Laser pulses created with a seed laser are stretched with the pulse stretcher, so that the energy density of the pulses is reduced. As an example, the pulse duration is increased from 250 fs to 450 ps. The stretched pulses are subjected to a high power amplification in the pulse amplifier. Because of the increased pulse duration, a distortion of amplifier media in the pulse amplifier due to optically non-linear effects can be avoided. As an example, an amplification by a factor 109 can be obtained using e.g. a Ti:sapphire amplifier. Finally, the amplified pulses are compressed in the pulse compressor, so that the pulse duration is reduced to the initial value of e.g. 250 fs.
Various types of pulse amplifiers have been proposed, like e.g. devices with optical parametric amplification (OPA) or regenerative amplifiers. As an example, T. Metzger et al. (“Optics Letters”, Vol. 34, 2009, p. 2123) have proposed a laser setup including a pulse amplifier with a fibre amplifier and a regenerative amplifier. The regenerative amplifier comprises an amplifier cavity with multiple cavity mirrors and a diode pumped disc amplifier. Furthermore, the amplifier cavity includes a Pockels cell arranged for switching selected pulses out of the amplifier cavity for subsequent pulse compression.
Generally, the conventional techniques suffer from severe accumulation of the B-integral (integrated non-linear phase shift) inside the laser gain medium and other optical components of the laser setup. The thin disc technology, as proposed e.g. by T. Metzger et al., reduced the accumulation of non-linear effects in the laser gain medium to a minimum. However, the Pockels cell for switching pulses in and out of the amplifier cavity still represents a limitation with regard to the obtainable amplification factor in the cavity. The crystal of the Pockels cell has a high optical non-linearity resulting in self-focusing and other non-linear effects, which deteriorate the pulse quality and limit the maximum pulse intensities. Thus, the needed optics inside the beam path of the laser setup (e.g. the Pockels cell) necessarily require the pulse stretcher, which typically includes large and complex dispersive delay lines for stretching the pulses to be able to amplify e.g. to the multi-milli-Joule level (see also Y. Akahane in “Optics Letters”, Vol. 32, 2007, p. 1899; K.-H. Hong et al. in “Optics Letters”, Vol. 35, 2010, p. 1752; and D. Nickel et al. in “Optics Letters, Vol. 29, 2004, p. 2764).
As a further problem, stretching pulses with spectral band-width of about 1 nm to 10 nm is expensive due to the costs of highly dispersive optics. Yet a further disadvantage of the conventional laser setups for providing high average power laser pulses results from the complex structure thereof. Multiple measures are necessary for keeping a sufficient mechanical stability of positioning and adjusting optical components within the laser setup.
The objective of the invention is to provide an improved laser pulse amplifier device and an improved method of amplifying laser pulses, respectively, being capable of avoiding limitations of conventional techniques. In particular, laser pulse amplification is to be provided with improved capability of increasing amplification factors, reduced complexity of the laser setup, improved pulse quality and/or reduced mechanical sensitivity.
These objectives are solved with a laser pulse amplifier device and a method of amplifying laser pulses comprising the features of the invention.